Re: Q: Powering a Logic Probe

Actually, I have to disagree. You should NOT connect a logic probe to
anything except the same power source as is powering the chips you are
testing.

In reality, the logic probe is designed to evaluate the VALID high and
low threshold of an output pin. The slide switch that says TTL/CMOS
is not setting up the voltage you wish to use (+5 or +12) but rather
what a valid HIGH or LOW signal should be since TTL differs slightly
from CMOS.

For CMOS, a low is defined as 1/3 or lower of the power supply. A
high is defined as 2/3rds or higher of the power supply. For TTL, a
valid low is below 0.8VDC, and a High is 2.8VDC or higher - this based
on 5.00 VDC. Values in between are considered invalid and will not be
displayed on either the high or low LED. Obviously, if you are using
+9VDC as the reference voltage instead of 5V, the high will be in the
wrong place. So, just using a common ground is not enough for a
proper test.

If all you are doing is trying to see "activity" then it might work in
a pinch. But, if you really need to probe a problem board, and you
can't find a decent place to attach for +5 and Ground, just solder a
couple of little loop eyelets which can be easily removed after you
solve your problem.

Can you use an external power source set for almost the exact same
voltage as the board? Absolutely!

RB

On Mar 9, 11:28 am, "cody chunn" <cchu...@xxxxxxxxxxx> wrote:
Steve,

Unless I'm mistaken (definitely possible), depending on what voltage circuit
you're probing, you need to hook the red clip to B+ for that voltage. For
instance, if you're probing a 12V sound circuit, you need to have the red
lead on + or - 12V, not 5V and vise versa.

--
-cody
--CARGPB4

"Steve Kulpa" <steveku...@xxxxxxxxx> wrote in message

news:f4e12cdc-a9f2-4962-953c-2468b3b5d5ac@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Mar 9, 12:10 pm, David Gersic <usenet_spam_t...@zaccaria-

pinball.com> wrote:
On Mon, 9 Mar 2009 05:53:27 -0700 (PDT), Steve Kulpa
<steveku...@xxxxxxxxx> wrote:
Do I have to hook the +/- 5V clips on my logic probe to the voltage
sources on the board I'm testing, or can I connect it to an external
power source?

Does your probe have its own ground reference? I have the cheap Radio
Shack
probe, and power it from an external source, but it has its own ground
reference clip that goes to the board being tested.

Nope, just + and - 5V clips.
I hadn't thought of hooking it up to the board's power supply.  That's
a good idea and easily done on my test setup.
No more looking for test points, etc...!

thanks again!
steve
---
Steve Kulpa (cargpb10)
Nolensville, TNhttp://www.geocities.com/stevekulpa/pinball.htm- Pinballhttp://www.geocities.com/stevekulpa/faces/rgpidx.htm - Faces

.

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